Variable-power optical system and imaging apparatus

ABSTRACT

The variable-power optical system is provided and includes a first lens group having a negative refractive power, a stop, and a second lens group having a positive refractive power, in order from an object side. The first lens group includes a first sub lens group having three negative meniscus lenses and a second sub lens group having a biconcave lens and a positive lens, in order from the object side. The second lens group includes a first positive lens arranged closest to the object side and having at least one aspheric surface and a second positive lens arranged immediately after the image side of the first positive lens. When the absolute value of the focal length of the first lens group is |f 1 | and the focal length of the entire system at a wide angle end is fw, the Conditional expression: 1.9&lt;|f 1 |/fw&lt;3.6 is satisfied.

This application is based on and claims priority under 35 U.S.C §119from Japanese Patent Application No. 2008-170863 filed on Jun. 30, 2008,the entire disclosure of which is herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a variable-power optical system usedfor, for example, a video camera or an electronic still camera and animaging apparatus, and more particularly, to a variable-power opticalsystem suitable to be used for a monitoring camera and an imagingapparatus including the variable-power optical system.

2. Description of the Related Art

Monitoring cameras have been used to operate unmanned facilities. Inrecent years, particularly, there is an increasing demand for monitoringcameras capable of changing power. A fast optical system having a largeaperture ratio needs to be used as a variable-power optical system forthe monitoring camera such that it can specify a subject even underlow-brightness imaging conditions. In addition, an optical systemapplied to the monitoring camera needs to have a compact structure and ahigh optical performance.

For example, JP-A-2006-119574 and JP-A-2007-94174 disclosevariable-power optical systems that have a large aperture ratio, a smallsize, and a high optical performance and can be mounted to themonitoring camera. JP-A-2006-119574 discloses an optical system thatincludes a first negative lens group, an aperture diaphragm, and asecond positive lens group arranged in this order from an object side.In the optical system, the first group includes three single lenses,that is, two negative lenses and one positive lens. JP-A-2007-94174discloses an optical system that includes a first negative lens groupand a second positive lens group arranged in this order from an objectside. In the optical system, the second group includes an aperturediaphragm, and a cemented lens of a negative meniscus lens and apositive lens is arranged closest to the object side in the secondgroup.

However, the optical system for the monitoring camera needs to have awide angle of view in order to monitor a wide range. In addition, inrecent years, there is an increasing demand for a monitoring cameracapable of obtaining a high-quality image. Therefore, there is anincreasing demand for a variable-power optical system capable ofcorresponding to a camera including an imaging device provided with1,000,000 pixels or more. However, in the optical system according tothe related art, it is difficult to achieve an optical performancecapable of corresponding to an increase in the number of pixels whilemaintaining a large aperture ratio required for the monitoring cameraand obtaining a wide angle of view.

SUMMARY OF THE INVENTION

The invention has been made in order to solve the above-mentionedproblems, and it is an object of the invention to provide avariable-power optical system that has a small size, a large apertureratio, a wide angle of view, and a high optical performance capable ofobtaining a high-quality image, and an imaging apparatus including thevariable-power optical system.

According to an aspect of the invention, a variable-power optical systemincludes a first lens group having a negative refractive power, a stop,and a second lens group having a positive refractive power arranged inthis order from an object side. A gap between the first lens group andthe second lens group on an optical axis is changed to vary power. Thefirst lens group is moved along the optical axis to correct the positionof an imaging surface after the variation in power. The first lens groupincludes a first sub lens group having three negative meniscus lensesand a second sub lens group having a biconcave lens and a positive lensarranged in this order from the object side. The second lens groupincludes a first positive lens that is arranged closest to the objectside and has at least one aspheric surface and a second positive lensthat is arranged immediately after the image side of the first positivelens. When the absolute value of the focal length of the first lensgroup is |f1| and the focal length of the entire system at a wide angleend is fw, the variable-power optical system satisfies Conditionalexpression 1 given below:

1.9<|f1|/fw<3.6.   [Conditional expression 1]

Here, ‘the second positive lens arranged immediately after the imageside of the first positive lens’ does not mean the distance relationshipbetween the first positive lens and the second positive lens, but meansthat there is no optical component with power between the first positivelens and the second positive lens.

In the variable-power optical system according to the above-mentionedaspect, since the first sub lens group including at least three negativemeniscus lenses is arranged close to the object side, it is possible toachieve a wide angle of view while preventing distortion. In addition,since an aspheric lens is arranged closest to the object side lens inthe second lens group, it is easy to correct spherical aberration thattends to increase with an increase in aperture ratio. Therefore, it iseasy to achieve a large aperture ratio. Further, since the aspheric lensmakes it easy to correct aberrations other than the sphericalaberration, it is easy to obtain a high optical performance whilereducing the size of an optical system. Furthermore, since thevariable-power optical system according to the above-mentioned aspectsatisfies Conditional expression 1, it is possible to maintain the powerratio of the first lens group to the entire optical system within anappropriate range. Therefore, it is possible to prevent the sphericalaberration at a telephoto end while reducing the size of an opticalsystem. As a result, it is possible to achieve an optical system thathas a large aperture ratio and is capable of a high-quality image.

In the variable-power optical system according to the above-mentionedaspect, when the average of the refractive indexes of all the negativemeniscus lenses included in the first sub lens group at the d-line is N1m, the variable-power optical system may satisfy Conditional expression2 given below:

N1m>1.70.   [Conditional expression 2]

In the variable-power optical system according to the above-mentionedaspect, when the Abbe number of the positive lens included in the secondsub lens group at the d-line is v2 p, at least one of the positivelenses may satisfy Conditional expression 3 given below:

v2p<20.0.   [Conditional expression 3]

In the variable-power optical system according to the above-mentionedaspect, the first lens group may include five single lenses, that is,three negative meniscus lenses, a biconcave lens, and a positive lensarranged in this order from the object side.

In the variable-power optical system according to the above-mentionedaspect, the second lens group may include four lenses, that is, thefirst positive lens, which is a biconvex lens, the second positive lens,which is a biconvex lens, a negative meniscus lens having a concavesurface facing an image side, and a biconvex lens arranged in this orderfrom the object side.

In the variable-power optical system according to the above-mentionedaspect, when the refractive index of the negative meniscus lens of thesecond lens group at the d-line is N23, the variable-power opticalsystem may satisfy Conditional expression 4 given below:

N23>1.95.   [Conditional expression 4]

The variable-power optical system according to the above-mentionedaspect may further include a third lens group that has a negativerefractive power, is provided on the image side of the second lensgroup, and is fixed when power varies.

In the variable-power optical system according to the above-mentionedaspect, at least one of the negative meniscus lenses included in thefirst sub lens group may have at least one aspheric surface.

According to another aspect of the invention, an imaging apparatusincludes the variable-power optical system.

According to the variable-power optical system of the invention, a firstlens group having a negative refractive power, an aperture diaphragm,and a second lens group having a positive refractive power are arrangedin this order from the object side. The first lens group includes atleast three negative meniscus lenses arranged close to the object side,and an aspheric lens is arranged closest to the object side in thesecond lens group. The structure of each lens group is appropriately setso as to satisfy Conditional expression 1. Therefore, it is possible toachieve an optical system that has a small size, a large aperture ratio,a wide angle of view, and a high optical performance capable ofobtaining a high-quality image.

An imaging apparatus according to the invention includes thevariable-power optical system according to the invention. Therefore, theimaging apparatus has a small size and a wide angle of view, and cancapture a high-quality image even under low-brightness imagingconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 1 of the invention;

FIG. 2 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 2 of the invention;

FIG. 3 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 3 of the invention;

FIG. 4 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 4 of the invention;

FIG. 5 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 5 of the invention;

FIG. 6 is a cross-sectional view illustrating the structure of lenses ofa variable-power optical system according to Example 6 of the invention;

FIGS. 7A to 7F are diagrams illustrating aberrations of thevariable-power optical system according to Example 1 of the invention;

FIGS. 8A to 8F are diagrams illustrating aberrations of thevariable-power optical system according to Example 2 of the invention:

FIGS. 9A to 9F are diagrams illustrating aberrations of thevariable-power optical system according to Example 3 of the invention;

FIGS. 10A to 10F are diagrams illustrating aberrations of thevariable-power optical system according to Example 4 of the invention;

FIGS. 11A to 11F are diagrams illustrating aberrations of thevariable-power optical system according to Example 5 of the invention;

FIGS. 12A to 12F are diagrams illustrating aberrations of thevariable-power optical system according to Example 6 of the invention;and

FIG. 13 is a cross-sectional view schematically illustrating an imagingapparatus according to an embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, exemplary embodiments of the invention will be described indetail with reference to the accompanying drawings.

FIG. 1 is a cross-sectional view illustrating the structure of avariable-power optical system according to an embodiment of theinvention. The structure shown in FIG. 1 corresponds to a variable-poweroptical system according to Example 1, which will be described below. InFIG. 1, the left side is an object side, and the right side is an imageside. FIG. 1 shows the arrangement of lenses at a wide angle end duringinfinity focusing. In FIG. 1, the movement locus of each lens group whenpower varies from a wide angle end to a telephoto end is schematicallyshown below the arrangement, which is represented by an arrow.

The variable-power optical system includes a first lens group G1 havinga negative refractive power, an aperture stop St, and a second lensgroup G2 having a positive refractive power arranged in this order fromthe object side along an optical axis Z. Such a structure having a lenswith negative power at its head is suitable to obtain a wide angle ofview, and can relatively easily ensure back focus.

The aperture stop St shown in FIG. 1 does not necessarily indicate asize or a shape, but indicates a position on the optical axis Z.

In FIG. 1, an imaging device 5 provided on an imaging surface of thevariable-power optical system is also shown, considering a case in whichthe variable-power optical system is applied to an imaging apparatus.The imaging device 5 is for capturing the image of a subject formed bythe variable-power optical system, and the imaging device 5 is arrangedsuch that an imaging surface thereof is disposed on an image formingsurface of the variable-power optical system.

When the variable-power optical system is applied to an imagingapparatus, it is preferable that, for example, cover glass or variousfilters, such as an infrared cut filter and a low pass filter, bearranged between the optical system and the imaging forming surface(imaging surface) according to the structure of a camera having lensesmounted thereto. FIG. 1 shows an example in which a parallel plateoptical part PP is arranged between the second lens group G2 and theimaging device 5, considering the above-mentioned structure.

The variable-power optical system changes the gap between the first lensgroup G1 and the second lens group G2 on the optical axis Z to changepower, and moves the first lens group G1 along the optical axis Z tocorrect the position of the imaging surface with the change in power.When power varies from the wide angle end to the telephoto end, thefirst lens group G1 and the second lens group G2 are moved so as to drawthe loci represented by solid arrows of FIG. 1. In addition, thevariable-power optical system moves the first lens group G1 along theoptical axis Z to perform focusing. The first lens group G1 also servesas a focus group.

The first lens group G1 includes a first sub lens group G11 and a secondsub lens group G12 arranged in this order from the object side. Thefirst sub lens group G11 includes at least three negative meniscuslenses, and the second sub lens group G12 includes a biconcave lens anda positive lens arranged in this order from the object side.

For example, as shown in FIG. 1, the first lens group G1 may includefive single lenses, that is, three lenses L11, L12, and L13, which arenegative meniscus lenses, a lens L14, which is a biconcave lens, and alens L15, which is a positive lens, arranged in this order from theobject side. In this example, the first sub lens group G11 includes thelenses L11, L12, and L13, and the second sub lens group G12 includes thelenses L14 and L15.

Since the negative meniscus lens is arranged closest to the object sidein the lens system, it is easy to correct spherical aberration at thetelephoto end, and it is effective to obtain a wide angle of view. Inaddition, the variable-power optical system according to this embodimentis characterized in that it includes at least three negative meniscuslenses. Therefore, it is possible to obtain the following effects thathave not been obtained in the related art.

In the optical system disclosed in JP-A-2006-119574, one negativemeniscus lens is arranged closest to the object side in the first lensgroup. However, since only one negative meniscus lens is provided,distortion increases with an increase in the angle of view of theoptical system. In contrast, in the variable-power optical systemaccording to this embodiment, since at least three negative meniscuslenses are arranged closest to the object side in the first lens groupG1, it is possible to obtain a wide angle of view while preventingdistortion.

In addition, at least one negative meniscus lens included in the firstsub lens group G11 of the first lens group G1 may have at least oneaspheric surface. In this case, it is possible to accurately correct allaberrations.

The second sub lens group G12 of the first lens group G1 includes anegative biconcave lens, which is shown as the lens L14 in FIG. 1.Therefore, it is easy to ensure negative power required for the firstlens group G1. As a result, it is possible to effectively correctaberration while decreasing the number of lenses to reduce the size ofan optical system.

Since the first lens group G1 includes a positive lens which is shown asthe lens L15 in FIG. 1, the positive lens can converge a beam divergedfrom the object side and guide the converged beam to the positive secondlens group. In addition, it is easy to keep the balance of aberration ofthe first lens group G1.

The second lens group G2 includes a first positive lens that is arrangedclosest to the object side and has at least one aspheric surface, and asecond positive lens that is arranged immediately after the image sideof the first positive lens.

For example, as shown in FIG. 1, the second lens group G2 includes foursingle lenses, that is, a lens L21, which is a biconvex positive lens,having at least one aspheric surface, a lens L22, which is a biconvexpositive lens, a lens L23, which is a negative meniscus lens having aconcave surface facing the image side, and a biconvex lens L24 arrangedin this order from the object side.

In the second lens group G2, since the lens L21 arranged closest to theobject side is an aspheric lens, it is easy to correct all aberrationsof a beam passing through a peripheral portion of the lens. Inparticular, it is easy to correct spherical aberration occurring due toa large aperture ratio, and it is easy to ensure a large aperture ratio.If the lens L21 is a spherical lens, positive power is increased as thedistance from the optical axis to a peripheral portion is increased. Asa result, there is a concern that spherical aberration will beexcessively corrected at the peripheral portion. In contrast, in thisembodiment, the lens L21 is an aspheric lens, and the lens L21 is formedin a shape in which the positive power is decreased as the distance fromthe optical axis to the peripheral portion is increased, as shown inFIG. 1. In this case, it is possible to prevent spherical aberrationfrom being excessively corrected, and thus reduce the sphericalaberration. Therefore, it is easy to obtain a large aperture ratio.

In addition, since the lens L21 is an aspheric lens, it is easy tocorrect all aberrations other than the spherical aberration. Therefore,it is possible to easily ensure a high optical performance whilereducing the size of an optical system. In particular, as shown in FIG.1, since an object-side surface S12 and an image-side surface S13 of abiconvex lens are aspheric surfaces, it is possible to easily ensure ahigh aberration correcting effect while reducing the size of an opticalsystem. The biconvex shape of the lens L21 is determined such that theobject-side surface has a convex shape with a large curvature toconverge a beam and the image-side surface has a convex shape to correctchromatic aberration.

As shown in FIG. 1, when all the positive lenses included in the secondlens group G2 are biconvex lenses, it is easy to ensure positive powerrequired to converge a beam diverged by the first lens group G1.Therefore, it is possible to effectively correct aberration whiledecreasing the number of lenses to reduce the size of an optical system.

The variable-power optical system having the above-mentioned structurecan easily obtain a wide angle of view, maintain a large aperture ratio,and achieve a high optical performance capable of obtaining ahigh-quality image with a compact structure. For example, thevariable-power optical system shown in FIG. 1 includes a total of ninelenses, that is, five lenses included in the first lens group G1 andfour lenses included in the second lens group G2. Therefore, thevariable-power optical system has a compact structure.

However, in the variable-power optical system according to thisembodiment of the invention, the number of lenses and the shapes of thelenses are not limited to those shown in FIG. 1, but variousmodifications thereof can be made. For example, the variable-poweroptical system according to this embodiment of the invention may furtherinclude a third lens group that has a negative refractive power and isfixed when power varies on the image side of the second lens group G2.The addition of the third lens group makes it possible to change animage size and correspond to a change in the size of an imaging device.Specifically, this structure can correspond to a change in the size ofthe imaging device, such as a change in the size of a CCD from 6 mm to 8mm.

The variable-power optical system according to this embodiment of theinvention may further include the following preferred aspects, inaddition to the above-mentioned structure, thereby obtaining a higheroptical performance.

When the absolute value of the focal length of the first lens group G1is |f1| and the focal length of the entire optical system at the wideangle end is fw, it is preferable that the variable-power optical systemsatisfy Conditional expression 1 given below:

1.9<|f1|/fw<3.6.   [Conditional expression 1]

Conditional expression 1 relates to the ratio of the focal length of thefirst lens group G1 to the focal length of the entire optical system atthe wide angle end, that is, the ratio of the power of the first lensgroup G1 to the power of the entire optical system. When the ratio isgreater than the upper limit of Conditional expression 1, the negativerefractive power of the first lens group G1 is reduced, and the movementof the first lens group G1 is increased as power varies, which resultsin an increase in the size of the optical system. When the ratio is lessthan the lower limit of Conditional expression 1, the negativerefractive power of the first lens group G1 is increased, and thespherical aberration is not sufficiently corrected at the telephoto end.

Examples 1 to 4 disclosed in JP-A-2007-94174 do not all satisfyConditional expression 1, and have values less than the lower limit ofConditional expression 1. In contrast, the variable-power optical systemaccording to this embodiment satisfies Conditional expression 1.Therefore, it is possible to reduce the size of an optical system andprevent an increase in spherical aberration at the telephoto end. As aresult, it is possible to obtain a high optical performance.

Further, it is preferable that the variable-power optical system satisfythe following Conditional expression 1-1 in order to obtain a highoptical performance while further reducing the size of the opticalsystem:

2.4<|f1|/fw<3.2.   [Conditional expression 1-1]

When the average of the refractive indexes of all the negative meniscuslenses in the first sub lens group G11 at the d-line is N1 m, it ispreferable that the variable-power optical system satisfy Conditionalexpression 2 given below:

N1m>1.70.   [Conditional expression 2]

When the average is less than the lower limit of Conditional expression2, the negative refractive power of the first lens group G1 is reduced,which results in an increase in the size of the optical system.

It is preferable that the variable-power optical system satisfy thefollowing Conditional expression 2-1 in order to further reduce the sizeof the optical system:

N1m>1.84.   [Conditional expression 2-1]

When the Abbe number of the positive lens included in the second sublens group G12 at the d-line is v2 p, it is preferable that at least onepositive lens of the variable-power optical system satisfy Conditionalexpression 3 given below:

v2p<20.0.   [Conditional expression 3]

When the Abbe number is greater than the upper limit of Conditionalexpression 3, chromatic aberration on the axis is increased at thetelephoto end.

When the second lens group G2 includes a negative meniscus lens having aconcave surface facing the image side and the refractive index of thenegative meniscus lens at the d-line is N23, it is preferable that thevariable-power optical system satisfy Conditional expression 4 givenbelow:

N23>1.95.   [Conditional expression 4]

When the refractive index is less than the lower limit of Conditionalexpression 4, the curvature of the image-side concave surface of thenegative meniscus lens is increased, which results in an increase infield curvature. Therefore, it is difficult to maintain a high opticalperformance in the range from the center of a screen to the periphery ofthe screen.

For example, when the variable-power optical system is used in a severeenvironment, such as outside, it is preferable that the lens arrangedclosest to the object side be made of a material capable of preventingthe deterioration of the surface of the lens due to rain and wind and atemperature variation due to direct lays of the sun, and having highresistance to chemicals, such as oils, fats, and a detergent, that is, amaterial having high water resistance, high weather resistance, highacid resistance, and high chemical resistance. In addition, it ispreferable that the lens be made of a hard and splinterless material.Specifically, it is preferable that the lens arranged closest to theobject side be made of glass or transparent ceramics.

It is preferable that the lens having the aspheric shape be made ofplastic. In this case, it is possible to accurately form an asphericshape, and reduce the weight and manufacturing costs of an opticalsystem.

When the variable-power optical system is required to be used in a widetemperature range, it is preferable that each lens be made of a materialwith a small linear expansion coefficient. In addition, when thevariable-power optical system is used in a severe environment, aprotective multi-layer film may be coated. In addition to the coating ofthe protective film, an antireflection film for reducing ghost light inuse may be coated.

In the example shown in FIG. 1, the optical part PP is arranged betweenthe lens system and the imaging surface. However, instead of providingvarious filters, such as a low pass filter and a filter for cutting aspecific wavelength band, various filters may be provided between thelenses, or a film having the same function as various filters may becoated on the lens surface of any lens.

As described above, according to the variable-power optical system ofthis embodiment, it is possible to achieve a wide angle of view whilemaintaining a compact structure and a large aperture ratio byappropriately using the above-mentioned preferred structure according torequired specifications. Therefore, it is possible to easily obtain ahigh-quality image corresponding to a camera including an imaging deviceprovided with 1,000,000 pixels or more.

EXAMPLE

Next, detailed numeric examples of the variable-power optical systemsaccording to the invention will be described.

Example 1

The cross-sectional view of lenses according to Example 1 is shown inFIG. 1. Specifically, a first lens group G1 of a variable-power opticalsystem according to Example 1 includes meniscus-shaped negative lensesL11, L12, and L13 having convex surfaces facing the object side, abiconcave negative lens L14, and a plano-convex positive lens L15 havinga convex surface facing the object side arranged in this order from theobject side. The second lens group G2 includes a positive lens L21having a biconvex shape in the vicinity of the optical axis, a biconvexpositive lens L22, a meniscus-shaped negative lens L23 having a convexsurface facing the object side, and a biconvex positive lens L24arranged in this order from the object side. In the variable-poweroptical system according to Example 1, an object-side surface S12 and animage-side surface S13 of the lens L21 are aspheric surfaces. Theposition of the aperture stop St is fixed, but the diameter thereof isvariable when power varies.

Lens data of the variable-power optical system according to Example 1 isshown in Table 1, aspheric data thereof is shown in Table 2, and variousdata is shown in Table 3. In addition, the meaning of symbols in thefollowing Tables 1 to 3 is the same as that in the following Examples.

In the lens data of Table 1, Si indicates an i-th (i=1, 2, 3, . . . )surface number. In this case, the surface of a component closest to theobject side is given number 1, and the surface number is sequentiallyincreased toward the image side. Ri indicates the curvature radius ofthe i-th surface, and Di indicates the gap between the i-th surface andan (i+1)-th surface on the optical axis Z. In addition, Ndj indicatesthe refractive index of a j-th (j=1, 2, 3, . . . ) optical componentwith respect to the d-line (wavelength: 587.6 nm). In this case, a lensarranged closest to the object side is given number 1, and the number issequentially increased toward the image side. In addition, vdj indicatesthe Abbe number of the j-th optical component with respect to thed-line. The lens data also includes the aperture stop St and the opticalpart PP. In the lens data, when the lens surface is convex toward theobject side, the curvature radius thereof has a positive value. When thelens surface is convex toward the image side, the curvature radiusthereof has a negative value.

In the lens data shown in Table 1, symbol * is added to the surfacenumber of an aspheric surface, and the curvature radius of the asphericsurface is represented by the value of a paraxial curvature radius. Theaspheric data shown in Table 2 indicates aspheric coefficients relatedto the aspheric surfaces. The aspheric coefficients are the values ofcoefficients K and Bm (m=3, 4, 5, . . . ) in an aspheric surfaceexpression represented by Expression A given below:

Zd=C·h ²/{1+(1−K·C ² ·h ²)^(1/2) }+ΣBm·h ^(m),   [Expression A]

(where Zd: the depth of an aspheric surface (the length of aperpendicular line that drops from a point on the aspheric surface at aheight h to a plane vertical to the optical axis that is tangent to thetop of the aspheric surface), h: height (the distance from the opticalaxis to a lens surface), C: the inverse number of a paraxial curvatureradius, and K and Bm: aspheric coefficients (m=3, 4, 5, . . . , 20)).

In the lens data shown in Table 1, a variable spacing D1, a variablespacing D2, and a variable spacing D3 are written in surface spacingfields corresponding to the gap between the first lens group G1 and theaperture stop St, the gap between the aperture stop St and the secondlens group G2, and the gap between the second lens group G2 and theoptical part PP that are changed to vary power.

Various data shown in Table 3 includes the focal length of the entireoptical system, an F value (Fno.), the entire angle of view, and thevalues of the variable spacing D1, the variable spacing D2, and thevariable spacing D3 at the wide angle end and the telephoto end. In thelens data and various data, the unit of length is millimeter. However,the optical system has the same optical performance even whenproportional magnification or proportional reduction is performed.Therefore, the unit of length is not limited to millimeter, but otherappropriate units may be used.

TABLE 1 Example 1 Lens data Si Ri Di Ndj νdj  1 21.3671 1.10 1.8348142.7  2 10.6261 2.23  3 14.9132 0.95 1.88300 40.8  4 8.4452 2.79  519.7316 0.87 1.88300 40.8  6 9.7910 2.93  7 −38.6003 3.74 1.80400 46.6 8 18.4696 1.37  9 20.6244 2.92 1.92286 18.9 10 ∞ Variable spacing D1 11(aperture stop) — Variable spacing D2 12* 13.2894 5.00 1.56883 56.3 13*−23.0141 0.89 14 14.0731 3.79 1.49700 81.6 15 −13.9494 0.10 16 41.39530.65 2.00069 25.5 17 7.3588 0.46 18 10.5646 3.17 1.51633 64.1 19−20.2979 Variable spacing D3 20 ∞ 1.50 1.51633 64.1 21 ∞

TABLE 2 Example 1 Aspheric data S12 (twelfth surface) K B3 B4 B5 B60.00000E+00 1.08095E−04 −1.92549E−04   1.85737E−05 −1.83773E−06   B7 B8B9 B10 B11 −1.67736E−07   −9.10310E−09   1.67155E−09 −1.15204E−10  2.49218E−12 B12 B13 B14 B15 B16 6.18458E−13 1.01136E−13 1.41690E−145.77915E−16 1.03866E−16 B17 B18 B19 B20 8.64951E−18 4.26468E−193.77745E−20 −2.04645E−20 S13 (thirteenth surface) K B3 B4 B5 B60.00000E+00 1.13402E−04 1.25108E−04 9.31170E−06 9.77685E−07 B7 B8 B9 B10B11 −6.53401E−07   6.56800E−08 −1.42610E−09   1.16421E−10 1.14149E−11B12 B13 B14 B15 B16 1.17958E−12 1.16141E−13 2.07224E−14 −2.53220E−16  1.86787E−16 B17 B18 B19 B20 1.31494E−17 1.36290E−18 1.14002E−19−7.42899E−21

TABLE 3 Example 1 Various data Entire Variable Variable Variable Focalangle of spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.211.33 146.3 14.90 8.24 3.00 angle end Telephoto 5.87 2.13 57.7 1.59 1.599.65 end

Example 2

FIG. 2 is a cross-sectional view illustrating lenses according toExample 2. The basic lens structure of a variable-power optical systemaccording to Example 2 is similar to that according to Example 1 exceptthat a meniscus-shaped positive lens L15 having a convex surface facingthe object side is used in Example 2 instead of the plano-convex lensL15 according to Example 1. In addition, in the variable-power opticalsystem according to Example 2, an object-side surface S12 and animage-side surface of a lens L21 are aspheric surfaces.

Lens data of the variable-power optical system according to Example 2 isshown in Table 4, aspheric data thereof is shown in Table 5, and variousdata thereof is shown in Table 6.

TABLE 4 Example 2 Lens data Si Ri Di Ndj νdj  1 18.8544 1.67 1.8830040.8  2 9.6898 2.70  3 12.0747 1.07 1.88300 40.8  4 6.6167 3.26  514.7479 1.64 1.81600 46.6  6 9.5646 3.03  7 −31.7750 3.05 1.79952 42.2 8 11.3138 0.24  9 10.8296 1.55 1.92286 18.9 10 39.8198 Variable spacingD1 11 (aperture stop) — Variable spacing D2 12* 13.4759 4.93 1.5638460.7 13* −21.7202 0.85 14 14.0015 3.75 1.49700 81.6 15 −14.6216 0.10 1699.6937 0.65 2.00330 28.3 17 7.8269 0.50 18 9.3574 3.08 1.57135 53 19−21.9950 Variable spacing D3 20 ∞ 1.50 1.51633 64.1 21 ∞

TABLE 5 Example 2 Aspheric data S12 (twelfth surface) K B3 B4 B5 B60.00000E+00 1.22394E−04 −1.89270E−04   1.88796E−05 −1.80161E−06   B7 B8B9 B10 B11 −1.62004E−07   −8.18066E−09   1.77722E−09 −1.02954E−10  3.56884E−12 B12 B13 B14 B15 B16 6.97472E−13 1.02216E−13 1.34937E−145.02023E−16 6.25009E−17 B17 B18 B19 B20 1.62167E−17 −1.16728E−18−1.66021E−19 −2.47243E−20 S13 (thirteenth surface) K B3 B4 B5 B60.00000E+00 6.16264E−05 1.25613E−04 9.02996E−06 9.06119E−07 B7 B8 B9 B10B11 −6.63016E−07   6.42661E−08 −1.57622E−09   9.94129E−11 9.97829E−12B12 B13 B14 B15 B16 1.05375E−12 1.09930E−13 2.04023E−14 −3.75122E−16  1.27890E−16 B17 B18 B19 B20 3.29006E−18 1.34527E−18 2.51249E−19−8.47282E−22

TABLE 6 Example 2 Various data Entire Variable Variable Variable Focalangle of spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.261.33 147.8 9.75 9.69 3.00 angle end Telephoto 6.02 2.59 56.6 3.76 1.0411.65 end

Example 3

FIG. 3 is a cross-sectional view illustrating lenses according toExample 3. The basic lens structure of a variable-power optical systemaccording to Example 3 is the same as that according to Example 2. Inaddition, in the variable-power optical system according to Example 3,an object-side surface S12 and an image-side surface S13 of a lens L21are aspheric surfaces.

Lens data of the variable-power optical system according to Example 3 isshown in Table 7, aspheric data thereof is shown in Table 8, and variousdata thereof is shown in Table 9.

TABLE 7 Example 3 Lens data Si Ri Di Ndj νdj  1 21.8943 1.26 1.8348142.7  2 11.7335 2.92  3 14.6451 1.10 1.88300 40.8  4 10.1764 2.84  516.4676 0.95 1.88300 40.8  6 10.7445 4.17  7 −35.7810 3.10 1.80400 46.6 8 16.1017 1.81  9 21.6836 3.10 1.92286 18.9 10 604.6536 Variablespacing D1 11 (aperture stop) — Variable spacing D2 12* 10.7994 4.241.49700 81.6 13* −93.9686 0.78 14 10.0028 2.86 1.72916 54.7 15 −32.29760.32 16 36.8848 0.82 2.00069 25.5 17 6.3990 0.67 18 14.4120 1.58 1.6180063.4 19 −18.2038 Variable spacing D3 20 ∞ 1.50 1.51633 64.1 21 ∞

TABLE 8 Example 3 Aspheric data S12 (twelfth surface) K B3 B4 B5 B60.00000E+00 1.42339E−04 −1.92648E−04   1.79668E−05 −1.96683E−06   B7 B8B9 B10 B11 −1.81872E−07   −1.07471E−08   1.52696E−09 −1.29680E−10  1.29811E−12 B12 B13 B14 B15 B16 5.02428E−13 9.27482E−14 1.33641E−144.68051E−16 1.75699E−17 B17 B18 B19 B20 1.52034E−17 6.55289E−198.24566E−20 −2.58498E−20 S13 (thirteenth surface) K B3 B4 B5 B60.00000E+00 1.78558E−04 1.25369E−04 9.28153E−06 1.01005E−06 B7 B8 B9 B10B11 −6.48018E−07   6.65048E−08 −1.33908E−09   1.26638E−10 1.23257E−11B12 B13 B14 B15 B16 1.27699E−12 1.19744E−13 2.31475E−14 −5.37227E−16  2.52236E−16 B17 B18 B19 B20 3.02821E−18 1.08655E−18 1.24629E−19−8.39075E−21

TABLE 9 Example 3 Various data Entire Variable Variable Variable Focalangle of spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.211.33 146.2 20.13 7.64 3.00 angle end Telephoto 5.87 1.95 57.3 1.40 2.618.03 end

Example 4

FIG. 4 is a cross-sectional view illustrating lenses according toExample 4. The basic lens structure of a variable-power optical systemaccording to Example 4 is the same as that according to Example 2. Inaddition, in the variable-power optical system according to Example 4,an object-side surface S12 and an image-side surface S13 of a lens L21are aspheric surfaces.

Lens data of the variable-power optical system according to Example 4 isshown in Table 10, aspheric data thereof is shown in Table 11, andvarious data thereof is shown in Table 12.

TABLE 10 Example 4 Lens data Si Ri Di Ndj νdj  1 22.6668 1.10 1.7432049.3  2 10.5107 2.50  3 14.2655 1.10 1.72916 54.7  4 8.4469 3.01  520.6186 0.96 1.75500 52.3  6 9.0955 2.97  7 −39.4534 3.81 1.88300 40.8 8 18.7657 1.56  9 21.0326 2.94 1.92286 18.9 10 593.0192 Variablespacing D1 11 (aperture stop) — Variable spacing D2 12* 13.1879 5.001.51633 64.1 13* −22.8785 0.81 14 12.8396 3.71 1.49700 81.6 15 −15.32980.10 16 38.9170 0.65 2.00330 28.3 17 7.6036 0.54 18 10.7694 3.18 1.4874970.2 19 −16.9744 Variable spacing D3 20 ∞ 1.50 1.51633 64.1 21 ∞

TABLE 11 Example 4 Aspheric data S12 (twelfth surface) K B3 B4 B5 B60.00000E+00 1.16876E−04 −1.96860E−04   1.82714E−05 −1.84730E−06   B7 B8B9 B10 B11 −1.66369E−07   −8.74938E−09   1.71948E−09 −1.08203E−10  3.23459E−12 B12 B13 B14 B15 B16 7.10596E−13 1.10558E−13 1.52918E−147.10077E−16 9.63697E−17 B17 B18 B19 B20 8.17578E−18 8.37191E−213.43247E−20 −1.87125E−20 S13 (thirteenth surface) K B3 B4 B5 B60.00000E+00 1.25697E−04 1.29797E−04 9.51603E−06 9.77232E−07 B7 B8 B9 B10B11 −6.55098E−07   6.53666E−08 −1.46105E−09   1.11890E−10 1.09550E−11B12 B13 B14 B15 B16 1.12103E−12 1.10930E−13 1.97343E−14 −4.55564E−16  1.85969E−16 B17 B18 B19 B20 1.83389E−17 1.39513E−18 2.26193E−19−6.26961E−21

TABLE 12 Example 4 Various data Angle Variable Variable Variable Focalof spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.20 1.33146.2 14.94 8.26 3.00 angle end Telephoto 5.86 2.09 57.8 1.94 1.35 9.91end

Example 5

FIG. 5 is a cross-sectional view illustrating lenses according toExample 5. The basic lens structure of a variable-power optical systemaccording to Example 5 is similar to that according to Example 1 exceptthat a biconvex positive lens L15 is used in Example 5 instead of theplano-convex lens L15 according to Example 1. In addition, in thevariable-power optical system according to Example 5, an object-sidesurface S3 and an image-side surface S4 of a lens L12, and anobject-side surface S12 and an image-side surface S13 of a lens L21 areaspheric surfaces.

Lens data of the variable-power optical system according to Example 5 isshown in Table 13, aspheric data thereof is shown in Table 14, andvarious data thereof is shown in Table 15.

TABLE 13 Example 5 Lens data Si Ri Di Ndj νdj  1 20.1765 1.10 1.8348142.7  2 10.4650 2.28  3* 14.7312 1.11 1.88300 40.8  4* 8.1241 2.85  519.4521 0.95 1.88300 40.8  6 9.6912 2.92  7 −39.6839 3.76 1.80400 46.6 8 17.9064 1.34  9 20.2777 2.93 1.92286 18.9 10 −1980.4893 Variablespacing D1 11 (aperture stop) — Variable spacing D2 12* 13.7193 5.001.56384 60.7 13* −23.9587 0.92 14 13.9038 3.80 1.49700 81.6 15 −14.30980.10 16 30.4983 0.65 2.00069 25.5 17 7.3508 0.45 18 10.6952 3.14 1.4874970.2 19 −19.6277 Variable spacing D3 20 ∞ 1.50 1.51633 64.1 21 ∞

TABLE 14 Example 5 Aspheric data S3 (third surface) K B3 B4 B5 B61.00000E+00 1.25479E−05 2.63261E−06 2.98723E−07 3.51345E−08 B7 B8 B9 B10B11 2.93760E−09 2.63735E−10 1.59042E−11 8.96750E−13 1.42999E−14 B12 B13B14 B15 B16 −2.62719E−15   −8.72492E−17   1.15866E−16 1.06375E−184.99750E−18 B17 B18 B19 B20 1.69493E−18 −7.63607E−20 1.88484E−21−1.76211E−21 S4 (fourth surface) K B3 B4 B5 B6 1.00000E+00−5.51292E−06   −1.25074E−06   −1.28714E−07   −1.28303E−08   B7 B8 B9 B10B11 −8.75487E−10   −5.72206E−11   −9.54933E−13   3.77160E−13 7.47451E−14B12 B13 B14 B15 B16 1.20874E−14 1.48614E−15 1.75368E−16 1.32086E−174.24735E−18 B17 B18 B19 B20 7.81487E−19 −3.52864E−19 −2.14844E−20−6.31658E−22 S12 (twelfth surface) K B3 B4 B5 B6 0.00000E+00 1.11889E−04−1.93702E−04   1.84727E−05 −1.84079E−06   B7 B8 B9 B10 B11−1.67209E−07   −8.96922E−09   1.68936E−09 −1.12686E−10   2.75765E−12 B12B13 B14 B15 B16 6.50917E−13 1.04257E−13 1.45299E−14 5.90089E−161.04034E−16 B17 B18 B19 B20 8.38415E−18 3.41338E−19 5.36328E−20−2.00175E−20 S13 (thirteenth surface) K B3 B4 B5 B6 0.00000E+001.15043E−04 1.26074E−04 9.41671E−06 9.82025E−07 B7 B8 B9 B10 B11−6.53865E−07   6.55345E−08 −1.44622E−09   1.13618E−10 1.11189E−11 B12B13 B14 B15 B16 1.14405E−12 1.12814E−13 2.03552E−14 −2.89827E−16  1.78906E−16 B17 B18 B19 B20 1.33286E−17 1.37166E−18 1.15638E−19−8.92651E−21

TABLE 15 Example 5 Various data Angle Variable Variable Variable Focalof spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.16 1.33144.1 14.92 8.23 3.00 angle end Telephoto 5.75 2.11 58.6 1.40 1.67 9.56end

Example 6

FIG. 6 is a cross-sectional view illustrating lenses according toExample 6. A variable-power optical system according to Example 6includes a first lens group G1, an aperture stop St, a second lens groupG2, and a third lens group G3. The basis lens structures of the firstlens group G1 and the second lens group G2 of the variable-power opticalsystem according to Example 6 are the same as those according to Example2. The third lens group G3 of the variable-power optical systemaccording to Example 6 is a fixed lens group that has a negativerefractive power and does not move when power varies. The third lensgroup G3 includes two lenses, that is, a biconcave negative lens L31 anda biconvex positive lens L32. In Example 6, a variable spacing D3 is thegap between the second lens group G2 and the third lens group G3, unlikethe above-mentioned Examples. In the variable-power optical systemaccording to Example 6, an object-side surface S12 and an image-sidesurface S13 of a lens L21 are aspheric surfaces.

Lens data of the variable-power optical system according to Example 6 isshown in Table 16, aspheric data thereof is shown in Table 17, andvarious data thereof is shown in Table 18.

TABLE 16 Example 6 Lens data Si Ri Di Ndj νdj  1 22.3555 1.10 1.7725049.6  2 10.3707 2.32  3 13.5408 1.11 1.80610 40.9  4 9.2119 2.84  522.0033 0.95 1.83481 42.7  6 9.8981 2.95  7 −43.7206 3.74 1.80400 46.6 8 16.8156 1.29  9 17.5091 2.93 1.92286 18.9 10 119.6821 Variablespacing D1 11 (aperture stop) — Variable spacing D2 12* 15.5208 4.911.62230 53.2 13* −23.0179 0.73 14 17.4185 3.65 1.49700 81.6 15 −12.93840.10 16 68.4022 0.66 2.00069 25.5 17 8.7393 0.50 18 13.1033 3.00 1.4874970.2 19 −18.1878 Variable spacing D3 20 −39.7756 0.60 1.83400 37.2 2112.6481 0.58 22 19.6969 1.94 1.72825 28.5 23 −18.5569 1.00 24 ∞ 1.501.51633 64.1 25 ∞

TABLE 17 Example 6 Aspheric data S12 (twelfth surface) K B3 B4 B5 B60.00000E+00 1.27635E−04 −1.91886E−04   1.86163E−05 −1.82308E−06   B7 B8B9 B10 B11 −1.65910E−07   −8.87062E−09   1.69405E−09 −1.12796E−10  2.73575E−12 B12 B13 B14 B15 B16 6.54279E−13 1.04944E−13 1.48198E−147.89654E−16 1.21896E−16 B17 B18 B19 B20 9.18565E−18 6.70802E−199.24759E−20 −2.56006E−20 S13 (thirteenth surface) K B3 B4 B5 B60.00000E+00 1.12348E−04 1.24777E−04 9.34471E−06 9.78904E−07 B7 B8 B9 B10B11 −6.54142E−07   6.55343E−08 −1.44394E−09   1.14030E−10 1.11794E−11B12 B13 B14 B15 B16 1.15291E−12 1.16181E−13 2.02734E−14 −2.69148E−162.25854E−16 B17 B18 B19 B20 1.41335E−17 1.51171E−18 7.19957E−204.89494E−22

TABLE 18 Example 6 Various data Angle Variable Variable Variable Focalof spacing spacing spacing length Fno. view D1 D2 D3 Wide 2.89 1.78144.7 14.90 8.21 1.00 angle end Telephoto 7.68 2.97 57.6 2.52 1.01 8.20end

Table 19 shows values corresponding to Conditional expressions 1 to 4 inExamples 1 to 6. As can be seen from Table 19, Examples 1 to 6 allsatisfy Conditional expressions 1 to 4.

TABLE 19 Exam- Exam- Exam- Exam- Exam- Example 1 ple 2 ple 3 ple 4 ple 5ple 6 Conditional 2.8 2.1 3.5 2.8 2.9 2.3 expression 1 |f1|/fwConditional 1.87 1.86 1.87 1.74 1.87 1.80 expression 2 N1m Conditional18.9 18.9 18.9 18.9 18.9 18.9 expression 3 ν2p Conditional 2.00 2.002.00 2.00 2.00 2.00 expression 4 N23

FIGS. 7A to 7C are aberration diagrams illustrating sphericalaberration, astigmatism, and distortion of the variable-power opticalsystem according to Example 1 at the wide angle end. FIGS. 7D to 7F areaberration diagrams illustrating spherical aberration, astigmatism, anddistortion of the variable-power optical system according to Example 1at the telephoto end. Each of the aberration diagrams shows aberrationusing the d-line as a reference wavelength, and the spherical aberrationdiagram shows aberration with respect to the g-line (wavelength: 436 nm)and the C-line (wavelength: 656.3 nm). In the spherical aberrationdiagram, Fno. means the F value. In the astigmatism diagram and thedistortion diagram, o means a half angle of view. Similarly, FIGS. 8A to8F, FIGS. 9A to 9F, FIGS. 10A to 10F, FIGS. 11A to 11F, and FIGS. 12A to12F are diagrams illustrating the aberrations of the variable-poweroptical systems according to Example 2, Example 3, Example 4, Example 5,and Example 6, respectively.

As can be seen from the above-mentioned data, in Examples 1 to 6, thevariable-power optical system having a variable power ratio of about 2.7has a small structure, and the F value at the wide angle end is in therange of 1.33 to 1.78. That is, a fast lens system having a largeaperture ratio is obtained. In addition, the variable-power opticalsystem has the entire angle of view in the range of 144° to 148°, whichis a wide range, at the wide angle end, accurately corrects eachaberration, and has a high optical performance in the visible range bothat the wide angle end and the telephoto end.

FIG. 13 is a diagram schematically illustrating the structure of amonitoring camera according to another embodiment of the invention,which is provided with the variable-power optical system according tothe embodiment of the invention. A monitoring camera 10 shown in FIG. 13includes a lens device 6 and a camera body 7. A variable-power opticalsystem 1 is provided in the lens device 6. FIG. 13 schematicallyillustrates the variable-power optical system 1 including the first lensgroup G1, the aperture stop St, and the second lens group G2.

In addition, the imaging device 5 that captures the image of a subjectformed by the variable-power optical system 1 is provided in the camerabody 7. Examples of the imaging device 5 may include a CCD (chargecoupled device) that converts an optical image formed by thevariable-power optical system into electric signals and a CMOS(complementary metal oxide semiconductor). The imaging device 5 isarranged such that its imaging surface is aligned with the imagingsurface of the variable-power optical system 1.

An aperture diaphragm mechanism 8 that changes the diameter of theaperture stop St is provided above the lens device 6. A zoom knob 9 forchanging the power of the variable-power optical system 1 and a focusknob 11 for adjusting the focus of the variable-power optical system 1are provided below the lens device 6.

Since the variable-power optical system 1 according to the embodiment ofthe invention has the above-mentioned advantages, the imaging apparatusaccording to this embodiment can have a small size and capture ahigh-quality image in a wide range even under low-brightness imagingconditions.

Although the embodiments and examples of the invention have beendescribed above, the invention is not limited thereto. Variousmodifications and changes of the invention can be made without departingfrom the scope and spirit of the invention. For example, the curvatureradius, the surface spacing, the refractive index, and the Abbe numberof each lens component are not limited to the values described in theabove-mentioned numerical examples, but they may have other values.

In the above-described embodiment, the monitoring camera is given as anexample of the imaging apparatus, but the invention is not limitedthereto. For example, the invention can be applied to other imagingapparatuses, such as a television camera, a video camera, and anelectronic still camera.

1. A variable-power optical system comprising: a first lens group havinga negative refractive power; a stop; and a second lens group having apositive refractive power, wherein the first lens group, the stop, andthe second lens group are arranged in this order from an object side, agap between the first lens group and the second lens group on an opticalaxis is changed to vary power, the first lens group is moved along theoptical axis to correct the position of an imaging surface due to thevariation in power, the first lens group includes a first sub lens grouphaving three negative meniscus lenses and a second sub lens group havinga biconcave lens and a positive lens arranged in this order from theobject side, the second lens group includes a first positive lens thatis arranged closest to the object side and has at least one asphericsurface and a second positive lens that is arranged immediately afterthe image side of the first positive lens, and when the absolute valueof the focal length of the first lens group is |f1| and the focal lengthof the entire system at a wide angle end is fw, the variable-poweroptical system satisfies Conditional expression 1 given below:1.9<|f1|/fw<3.6.   [Conditional expression 1]
 2. The variable-poweroptical system according to claim 1, wherein, when the average of therefractive indexes of all the negative meniscus lenses included in thefirst sub lens group at a d-line is N1 m, the variable-power opticalsystem satisfies Conditional expression 2 given below:N1m>1.70.   [Conditional expression 2]
 3. The variable-power opticalsystem according to claim 1, wherein, when the Abbe number of thepositive lens included in the second sub lens group at the d-line is v2p, at least one of the positive lenses satisfies Conditional expression3 given below:v2p<20.0.   [Conditional expression 3]
 4. The variable-power opticalsystem according to claim 1, wherein the first lens group includes fivesingle lenses, that is, three negative meniscus lenses, a biconcavelens, and a positive lens arranged in this order from the object side.5. The variable-power optical system according to claim 1, wherein thesecond lens group includes four lenses, that is, the first positivelens, which is a biconvex lens, the second positive lens, which is abiconvex lens, a negative meniscus lens having a concave surface facingan image side, and a biconvex lens arranged in this order from theobject side.
 6. The variable-power optical system according to claim 5,wherein, when the refractive index of the negative meniscus lens of thesecond lens group at the d-line is N23, the variable-power opticalsystem satisfies Conditional expression 4 given below:N23>1.95.   [Conditional expression 4]
 7. The variable-power opticalsystem according to claim 1, further comprising: a third lens group thathas a negative refractive power, is provided on the image side of thesecond lens group, and is fixed when power varies.
 8. The variable-poweroptical system according to claim 1, wherein at least one of thenegative meniscus lenses included in the first sub lens group has atleast one aspheric surface.
 9. An imaging apparatus comprising thevariable-power optical system according to claim 1.